void PreviousButtonClicked(object sender, EventArgs e) { if (op1.Checked) { tl.CalculateUserMarks(1); } else if (op2.Checked) { tl.CalculateUserMarks(2); } else if (op3.Checked) { tl.CalculateUserMarks(3); } else if (op4.Checked) { tl.CalculateUserMarks(4); } Question question = tl.GetPreviousQuestion(); if (question != null) { DisplayQuestion(question); } }